Laser beams (hereinafter may be referred to as “laser”) have currently been utilized in various fields. In addition, research and development of laser beams has also increasingly been conducted. For example, a method of increasing output of pulse laser or a method of stabilizing the output has been studied.
MOPA (Master Oscillation Power Amplifier) has been known as a technique for increasing output of pulse laser. MOPA refers to a method for amplifying weak pulses from a laser oscillator (hereinafter may simply be referred to as an “oscillator”) using an amplifier or a system adopting such a method.
An amplifier used in MOPA generally amplifies light pulses with the use of a multi-pass amplification method or a regenerative amplification method. The multi-pass amplification method is a method for amplifying light pulses by causing light pulses to pass through a light amplification medium within an amplifier a plurality of times. Meanwhile, the regenerative amplification method is a method for amplifying light pulses by confining light pulses within an amplifier until a desired pass number is attained, for example, by controlling polarization.
In an example where a light pulse emitted from pulse laser has a pulse width not greater than several hundred nanoseconds, light amplification ends within an amplifier in a period of time not longer than a microsecond. Therefore, it is not easy to stabilize energy of light pulses output from a light amplifier.
Regarding a technique for stabilizing energy of a light pulse, for example, Non-Patent Document 1 (Strohkendl et al., “High stable amplification of femtosecond pulses,” J. Opt. Soc. Am. B, Vol. 11, No. 5, pp. 742-749 (1994)) discloses a method of stabilizing output energy by optimizing pass number in a MOPA system having a single multi-pass amplifier.
For example, Non-Patent Document 2 (Chu et al., “A versatile 10-TW laser system with robust passive controls to achieve high stability and spatiotemporal quality,” Appl. Phys. B79, pp. 193-201 (2004)) discloses a method of suppressing influence on an amplifier by fluctuation in output from an excitation light source by sufficiently saturating gain of the amplifier.
For example, Non-Patent Document 3 (Oksenhendler et al., “Femtosecond laser pulse energy self-stabilization,” Appl. Phys. B79, pp. 933-935 (2004)) discloses a method of detecting intensity of light pulses output from a pulse laser apparatus and adjusting energy of the pulses by using a light modulator operating at an ultra high speed.    Non-Patent Document 1: Strohkendl et al., “High stable amplification of femtosecond pulses,” J. Opt. Soc. Am. B, Vol. 11, No. 5, pp 742-749 (1994)    Non-Patent Document 2: Chu et al., “A versatile 10-TW laser system with robust passive controls to achieve high stability and spatiotemporal quality,” Appl. Phys. B79, pp 193-201 (2004)    Non-Patent Document 3: Oksenhendler et al., “Femtosecond laser pulse energy self-stabilization,” Appl. Phys. B79, pp 933-935 (2004)